1. Field of the Invention
The present invention relates generally to mechanical power transmission systems and more specifically to continuously variable drivetrains.
2. Related Art
Pulley and belt systems used to transmit mechanical energy are very common and have been used extensively in industry for decades. Their benefits of low cost, reliability, modularity, and high efficiency have created thousands of applications. Typically, two pulleys are used, although three, four, five, or more pulleys may be employed. Generally one belt or cable is used, although systems with multiple belts are not uncommon, such as those used in automobiles. With these systems, two or more pulleys have two or more annular grooves that are designed to accommodate two or more belts. Pulleys can be made from steel, aluminum, plastic, and other materials. The material choice is often determined by the amount of power to be transferred. Pulleys come in many different sizes, ranging from miniature pulleys with a diameter of less than 10 millimeters to very large pulleys over a meter in diameter. Belts are made from many different materials, but all of them are flexible. Often, a rubber belt is used with embedded steel strands to increase strength. Other common materials used in belt construction are urethane, neoprene, steel, and composites. The belt profile can be round, V shaped, flat, grooved, or other shapes. Timing belts use a series of tooth shaped ridges which engage corresponding indentations in a pulley to maximize power transfer and eliminate slip. Some belts employ grooves to allow them to wrap around smaller diameter pulleys.
Most pulley and belt drivetrains are endless, which means that they transfer power rotationally from one pulley to another. The pulleys are rigidly attached to rotating drive and driven shafts and a circular belt rotates endlessly in a closed loop. Idler pulleys are frequently used to create and maintain tension on the belt to prevent slippage and premature failure. Idler pulleys do not transfer power and typically employ a bearing in the bore of the pulley to minimize friction and increase life. The bearing and idler pulley assembly is often pressed over a non-rotating shaft.
Reciprocating pulley and belt, or cable, drivetrains are often found in human powered systems. Exercise equipment frequently uses a cable that is attached to weights at one end and to a bar or other device which a person can push or pull. The weight is lifted and then returned to its resting state. An idler pulley is generally suspended at a height above the weights. This lifting and lowering of the weight creates reciprocating motion of the cable and pulley. Similarly, exercise machines such as those simulating the motion of climbing stairs can use similar reciprocating pulley and cable drivetrains. All of these drivetrains suffer from a loss of kinetic energy at the end of each stroke. For example, in a bicep curl, the human grasps a bar with both hands and lifts the bar to a position near the chest, and then returns it to the resting state. Kinetic energy is created during movement of the bar and then lost when the movement is stopped at the end of the stroke. Some exercise machines, including Nautilus type equipment, employ a cam which causes the weights to move more rapidly at the end of the stroke. This effect creates more efficient exercise by minimizing the loss of kinetic energy. The exercise is also more efficient because it becomes more difficult as the muscle contracts. During contraction the mechanical advantage of a muscle increases and it becomes more powerful. As the muscle position changes and creates a larger mechanical advantage, with cam or Nautilus type equipment, the weight simultaneously becomes more difficult to lift.
Linear drive systems in human powered vehicles have been attempted many times. However, they are not as efficient as commonly used drivetrains, such as sprocket and chain systems used on bicycles, due to the loss of kinetic energy at the end of each stroke. Many of the human powered linear drive systems are also complex, and each gear, bearing, pulley, cable, chain, or sprocket used in the drivetrain reduces efficiency. The complex systems are also heavy, and weight is a significant factor in human powered vehicles because it increases inertia and power requirements. Complex systems are also more expensive and more prone to breaking.
The most common human powered vehicle is a bicycle. A bicycle uses a sprocket and chain drivetrain which very efficiently transfers human power to the rear wheel. However, power is only efficiently created through about 60 degrees of the stroke, and only becomes very efficient for about 30 degrees of the 360 degree rotary stroke. This stroke also creates two large torque spikes per revolution. In order to reduce stress on the body (especially the knees), and minimize fatigue, a high pedaling speed is required to achieve high efficiency. This high pedaling speed reduces the torque spikes and also creates momentum to carry the pedals through the power phase of the stroke. However, the majority of people are not comfortable pedaling at a high speed and consequently do not maintain a cadence which maximizes the efficiency inherent in a bicycle's rotary stroke.
Further, the most common complaint from individuals riding bicycles is discomfort created by the bike seat. This discomfort is significant enough to keep many people from riding bikes, and to reduce the frequency that others use their bicycles. Recent studies showing that bicycle riding contributes to impotence and other health problems aggravate the discomfort problem caused from bike seats. However, maximizing the efficiency inherent in the bicycle drivetrain requires that the user stay seated while pedaling. This position is more conducive to a higher cadence and expends less of the user's energy. Riding a bicycle seated creates a situation where most of the user's weight is on the seat, and thus prevents the majority of the user's weight from being applied to the pedals. This loss in force can only be regained by pedaling at high speed, where there is a corresponding drop in torque and less force needs to be applied to the pedals to maintain an efficient power output.
The second most common complaint among bicycle users is difficulty when shifting. While this is rarely a problem with avid cyclists, infrequent users routinely shift in the wrong direction, shifting to a higher gear when starting up a hill, or vice versa. This problem can lead to the chain coming off of a sprocket, binding of the chain, a broken chain, and in rare cases the user getting injured in a fall. The problem frustrates enough people that it reduces the percentage of the population that ride a bicycle.
There exists a need for a human powered drivetrain that eliminates the torque spike inherent in a bicycle drivetrain and that allows lower speed, efficient pedaling at a cadence comfortable for the majority of people. There also exists a need for a linear drivetrain that minimizes or eliminates the loss of kinetic energy at the end of each stroke. There exists a need for a simple, inexpensive, lightweight, and efficient linear drivetrain that can be altered to accommodate different user sizes and preferences. Additionally, there exists a need for human powered vehicles where discomfort from the seat is eliminated and that allows most or all of the user's weight to be applied to the pedals. Finally, there exists a need for a drivetrain which eliminates shifting of the derailleur system used to vary speed and torque on hills.